This invention relates to a piezoelectric transducer and a process for producing the same and more particularly relates to a piezoelectric transducer made from a thin film of lead titanate (PbTiO.sub.3) of excellent piezoelectricity and a process for producing the same.
It is heretofore known that lead titanate has a large axial ratio and spontaneous polarization Ps in the ferroelectric phase (tetragonal system).
It is generally believed that the piezoelectricity of a ferroelectric crystal is proportional to the magnitude of spontaneous polarization Ps. It was reported that in an experiment where a fine single crystal of lead titanate was used, the ferroelectric crystal actually had a large piezoelectricity.
However, because it is difficult in reality to produce lead titanate in the form of a single crystal of a practical size, lead titanate is used in the form of a ceramic at present. However, when lead titanate is formed into a ceramic, c-axes of the PbTiO.sub.3 crystallites are randomly oriented and therefore the merit of lead titanate that the piezoelectricity in the c-axis direction is large can not be fully utilized.
On the other hand, a piezoelectric transducer having a structure in which a metal film as an electrode is formed on a substrate and a piezoelectric thin film is deposited on it is also known. However, even in such piezoelectric thin films, only those made from limited piezoelectric materials can be used in actual piezoelectric transducers, because no satisfactory piezoelectric property can be attained by a polycrystalline film in which axes of the crystallites along which they show strong piezoelectricity are random. For example, a ZnO film deposited on a gold film by radio-frequency sputtering is uniformly oriented and shows piezoelectricity in the direction of the thickness of the film, so that it is used for a piezoelectric transducer. However, when a perovskite-type piezoelectric material is formed into a thin film by usual film forming techniques, the formed thin film is non-oriented. Therefore, it has been difficult to produce practical thin-film piezoelectric transducers in which the large piezoelectricity are fully utilized. A particular example (Japanese Patent Laid-Open No. 223230/1984) shows that a (100)-oriented platinum (Pt) film as an electrode is placed on a magnesium oxide (MgO) single crystal substrate and a c-axis-oriented thin film of lead titanate is then formed on the electrode.
In this process, however, it is only possible to use very limited kinds of single crystal substrates that have a lattice constant nearly equal to that of a piezoelectric substance.